1. Field of the Invention
The present invention relates to heat exchange vessels, such as nuclear steam generating vessels, and particularly relates to means for controlling the flow of heat exchange fluid along the outsides of a bundle of heat exchange tubes.
2. Description of the Prior Art
A typical nuclear steam generator comprises a vertically oriented shell or vessel and a plurality of inverted U-shaped tubes disposed in the shell so as to form a tube bundle. Each tube has a pair of elongated vertical portions interconnected at the upper end by a curved bight portion, so that the vertical portions of each tube straddle a center lane or passage through the tube bundle. A tube sheet supports the vertical portions of the tubes at their lower ends. The vertical tube portions on one side of the center lane communicate with a primary fluid inlet header and those on the other side of the center lane communicate with a primary fluid outlet header beneath the tube sheet. The steam generator also comprises a cylindrical wrapper sheet disposed between the tube bundle and the shell to form an annular chamber with the internal shell.
The primary fluid, having been heated by circulation through the reactor core, enters the steam generator through the primary fluid inlet header, is transmitted through the tube bundle and out the primary fluid outlet header. At the same time, a secondary fluid or feedwater is circulated around the tubes above the tube sheet in heat transfer relationship with the outside of the tubes, so that a portion of the feedwater is converted to steam, which is then circulated through standard electrical generating equipment. More particularly, the feedwater is conducted down the annular chamber along the outside of the wrapper to the tube sheet, and then upwardly among the tubes inside the wrapper.
The feedwater contains particles of material, mainly in the form of iron oxides and copper compounds, along with traces of other metals, which tend to settle out of the feedwater onto the tube sheet in those areas of the tube sheet where the velocity of lateral flow across the tube sheet is insufficient to prevent settling. This settling is harmful because it creates buildups of sludge deposits which provide sites for concentration of corrosive agents at the tube walls that result in tube corrosion. Since there are no tubes in the center lane, it is desirable to minimize flow along the center lane so as to increase lateral flow through the tube bundle, thereby reducing the size of the area on the tube sheet where the velocity of lateral flow is insufficient to prevent sludge settling.
To this end, some steam generator vessels have been provided with built-in tube lane blocks in the center lane to inhibit the flow of feedwater therealong. However, such built-in tube lane blocks have only been provided in nuclear steam generators of relatively recent vintage.
In the case of earlier steam generator vessels, it has been necessary to retrofit them with removable tube lane blocks. These blocks had to be inserted through available handholes in the vessel wall a slight distance above the tube sheet, the handholes typically being aligned with the center lane. The tube lane blocks are inserted in sections small enough to fit through the handhole, the blocks containing tabs adapted to interfit with saddles mounted on the tube sheet, the parts being inserted through a cutout in the wrapper. Split cover halves are then installed to cover the wrapper cutout, these cover halves being disposed in notches in the blocks to inhibit vertical movement thereof. The split cover halves are then secured to a sleeve fitted in the handhole and held in place by set screws. The entire assembly is then sealed by the handhole cover.
This arrangement provides effective tube lane blockage, but is extremely time consuming and expensive to install and to disassemble. Whenever it is desired to provide sludge lancing or other maintenance operations, such as foreign object search and retrieval, it is necessary to disassemble the tube block assembly by removing the sleeve and split cover halves one piece at a time, and then lifting the blocks from their notches and moving them out of the way. Considerable man/rem exposure is entailed. Furthermore, there is significant possibility that loose parts may be left in the generator. Installation and removal has to be done very carefully in order to avoid damaging the tubes.